The invention relates to the processing of light weight, bulky cellulosic material, such as straw and other non-wood cellulosic material, to pulp. The invention particularly relates to chemical processing of such material.
Straw and other light weight, bulky cellulosic material are converted to pulp for use in paper, building materials and other pulp based products. These materials are processed by chemical and mechanical processing treatments. The chemical treatment of these materials typically involves caustic chemicals and short processing times.
Chemical treatment vessels that treat straw and other light weight, bulky cellulosic materials accommodate the severe chemical conditions and short retention times involved in the chemical processing, e.g., hydrolysis, of these materials. A conventional chemical treatment vessel includes a series of horizontal tubes arranged side-by-side and is referred to as a Pandia digester. Conduits connect the tubes and provide a flow path for material flowing from the discharge of one tube to the inlet to the next tube. The arrangement of tubes requires a relatively complex mechanical assembly to support the Pandia digester. Material undergoing treatment flows from one tube to the next.
In the tubes, the material is maintained at temperatures of 200° C. and pressures of 20 bar (about 290 pounds per square inch (psi)) with retention times of less than 30 minutes. Screws internal to each tube move the material through each tube. The screws are prone to becoming clogged with the material and require maintenance.
The multiple tubes make the Pandia digester a mechanically complex device having a large number of moving components, e.g., screws. There is a long felt need for treatment vessels having few moving components, at least as compared to the multiple screw conveyors in a Pandia digester. There is also a long felt need for chemical treatment vessel capable of processing large volumes of material, such as 400 tons per day with a four minute retention time in the vessel. Accordingly, there is a long felt need for a chemical treatment vessel having a relatively simple structure and capable of processing large volumes of straw and other light weight bulky cellulosic materials.